Honeycomb core having a high compression strength

ABSTRACT

A method of making a fiber-reinforced composite structure comprising the steps of (i) forming a paper sheet having a Gurley air resistance at least 200 seconds per 100 milliliters, the sheet comprising from 30 to 70 weight percent p-aramid fiber, (ii) depositing on both surfaces of the paper sheet a compression enhancing layer in a quantity up to 5 weight percent based on the weight of the paper, (iii) forming a honeycomb from the sheet of step (ii), and (iv) applying a matrix resin coating to the honeycomb of step (iii).

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to a high compression strength honeycomb core anda method of making the core from a paper comprising p-aramid fiber andfibrids.

Description of Related Art

Core structures for sandwich panels from p-aramid fiber papers orwet-laid nonwovens, mostly in the form of honeycomb, are used indifferent applications but primarily in the aerospace industry wherestrength to weight or stiffness to weight ratios have very high values.Traditionally, such core structures have been optimized for maximumshear modulus (stiffness) of the core. For example, U.S. Pat. No.5,137,768 to Lin describes a honeycomb core made from high-density papercomprising 50 weight percent or more of p-aramid fiber in the form offloc (cut fiber) with the rest of the composition being a binder andother additives.

In many cases, improved core shear properties have been achieved,without noticeable improvement in core compressive strength. In someinstances, reduction in honeycomb core weight, while keeping the same orbetter shear properties, has led to some sacrifice in core compressionstrength. There is therefore an ongoing need for improved compressionstrength of honeycomb core based on p-aramid fiber paper.

BRIEF SUMMARY OF THE INVENTION

This invention pertains to a method of making a fiber-reinforcedcomposite structure comprising the steps of:

-   -   (i) forming a paper sheet having a Gurley air resistance at        least 200 seconds per 100 milliliters, the sheet comprising from        30 to 70 weight percent p-aramid fiber,    -   (ii) depositing on both surfaces of the paper sheet a        compression enhancing layer in a quantity up to 5 weight percent        based on the weight of the paper,    -   (iii) forming a honeycomb from the sheet of step (ii), and    -   (iv) applying a matrix resin coating to the honeycomb of step        (iii).

The invention also pertains to a honeycomb core comprising in order

-   -   (i) a paper sheet, the sheet comprising meta-aramid fibrids,        para-aramid fibrids or a combination thereof,    -   (ii) a compression enhancing layer deposited onto at both outer        surfaces of the paper sheet in a quantity up to 5 weight percent        based on the weight of the paper, and    -   (iii) a matrix resin coating resin coated onto the surface of        the compression enhancing layer, wherein the matrix coating        resin is phenolic, polyimide, polyetherimide, epoxy or        combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b are representations of views of a hexagonal shapedhoneycomb.

FIG. 2 is a representation of another view of a hexagonal cell shapedhoneycomb.

FIG. 3 is an illustration of honeycomb provided with facesheets.

FIG. 4 shows a cross section through a honeycomb cell wall of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

This invention pertains to a honeycomb core comprising a plurality ofinterconnected walls having surfaces that define a plurality ofhoneycomb cells, wherein the cell walls are formed from a paper which,prior to impregnation with a resin, comprises 30-70 parts by weightp-aramid fiber, and 30-70 parts by weight of aramid fibrids and has aGurley air resistance of at least 200 seconds per 100 milliliters.Further the surface of the paper is modified by a coating, a compressionenhancement layer, that subsequently contributes to providing improvedcompressive bond strength in a honeycomb structure.

More preferably, the cell walls are formed from a paper which, prior toimpregnation with a resin, comprises 50-70 parts by weight p-aramidfiber, and 30-50 parts by weight of aramid fibrids, has a Gurley airresistance of at least 500 seconds per 100 milliliters.

A surprising synergistic effect has been found between the material ofthe compression enhancing layer and a paper optimized to have a Gurleyair resistance of at least 200 seconds per 100 milliliters. Neither ofthese features, when taken alone contributes to enhanced compressionstrength but when combined, enhanced performance is observed. Gurley airresistance is a convenient measure of paper density (porosity). Alsosurprisingly, further improvement of compression strength of thehoneycomb core was found to be associated with increasing the paperdensity above 0.8 g/cm³.

Honeycomb

FIG. 1a is a plan view illustration of one honeycomb 1 of this inventionand shows cells 2 formed by cell walls 3. FIG. 1b is an elevation viewof the honeycomb shown in FIG. 1a and shows the two exterior surfaces,or faces 4 formed at both ends of the cell walls. The core also hasedges 5. FIG. 2 is a three-dimensional view of the honeycomb. Shown ishoneycomb 1 having hexagonal cells 2 and cell walls 3. The “T” dimensionor the thickness of the honeycomb is shown at 10 in FIG. 2. Hexagonalcells are shown; however, other geometric arrangements are possible withsquare, over-expanded and flex-core cells being among the most commonpossible arrangements. Such cell types are well known in the art andreference can be made to Honeycomb Technology by T. Bitzer (Chapman &Hall, publishers, 1997) for additional information on possible geometriccell types.

FIG. 3 shows a structural sandwich panel 5 assembled from a honeycombcore 6 with face sheets 7 and 8, attached to the two exterior surfacesof the core. The preferred face sheet material is a prepreg, a fibroussheet impregnated with thermoset or thermoplastic resin althoughmetallic face sheets may also be utilized. With metallic face sheets,and in some circumstances with prepreg, an adhesive film 9 is also used.Normally there are at least two prepreg skins on either side of thecore.

Processes for converting web substrates such as paper into honeycombcore are well known to those skilled in the art and include expansionand corrugation. The expansion process is particularly well suited formaking core from paper. Such processes are further detailed on page 721of the Engineered Materials Handbook, Volume 1—Composites, ASMInternational, 1988. The paper web can be coated with a matrix resinbefore or after formation of the honeycomb. Resin can be employed whichis cross-linked after application to the paper to optimize finalproperties such as stiffness and strength. Examples of resins includeepoxy, phenolic, acrylic, polyimide and mixtures thereof with phenolicbeing preferred. United States Military Specification MIL-R-9299Cspecifies appropriate resin properties. The final mechanical strength ofcore is result of a combination of several factors. The principal knowncontributors are paper composition and thickness, cell size, and finalcore density such as after coating with resin. Cell size is the diameterof an inscribed circle within the cell of a honeycomb core. For p-aramidcore, typical cell sizes range from ⅛″-¼″ (3.2 mm-6.2 mm) but othersizes are possible. Typical final core densities are in the range of38-96 kg/m³.

For the same cell size, the same final core density and the same resincontent, the core of the current invention has improved compressionstrength in comparison with other cores from p-aramid fiber papers knownin the art.

FIG. 4 shows generally at 40 a cross section through a honeycomb cellwall of this invention. The paper is shown at 41, the compressionenhancement layer at 42 and the matrix resin coating at 43.

This concept is also applicable to other core structures such as foldedcore. Examples of folded core are described in U.S. Pat. Nos. 6,935,997B2; 6,800,351 B1 and 6,913,570 B2.

Paper

The thickness of the paper used to make the core is dependent upon theend use or desired properties of the core structure and in someembodiments is from 0.6 to 20 mils (15 to 500 micrometers) thick. Morepreferably, the thickness is from 1.0 to 3.0 mils (25 to 75micrometers). In some embodiments, the basis weight of the paper is from0.5 to 6 ounces per square yard (15 to 200 grams per square meter).

In addition to p-aramid fibers, the paper of the core of this inventioncan also comprise, other fibers such as m-aramid, carbon, polybenzazole,polypyridazole, polyetherimide, polyphenylsulfone, liquid crystallinepolyester. Further, the paper can include inorganic particles such asmica, vermiculite, and the like; the addition of these performanceenhancing additives being to impart properties such as improved fireresistance, thermal conductivity, dimensional stability, and the like tothe paper and the final core structure.

The paper used to make the honeycomb core of this invention can beformed on equipment of any scale, from laboratory screens tocommercial-sized papermaking machinery, including such commonly usedmachines as Fourdrinier or inclined wire paper machines. A typicalprocess involves making a dispersion of fibrous material such as flocand/or pulp and fibrids in an aqueous liquid, draining the liquid fromthe dispersion to yield a wet composition and drying the wet papercomposition. The dispersion can be made either by dispersing the fibersand then adding the fibrids or by dispersing the fibrids and then addingthe fibers. The final dispersion can also be made by combining adispersion of fibers with a dispersion of the fibrids; the dispersioncan optionally include other additives such as inorganic materials. Theconcentration of fibers from the floc and pulp in the dispersion canrange from 0.01 to 1.0 weight percent based on the total weight of thedispersion. An example of a suitable range for fibrid concentration isthat it should be equal to or less than 30 weight percent based on thetotal weight of solids. In a typical process, the aqueous liquid of thedispersion is generally water, but may include various other materialssuch as pH-adjusting materials, forming aids, surfactants, defoamers andthe like. The aqueous liquid is usually drained from the dispersion byconducting the dispersion onto a screen or other perforated support,retaining the dispersed solids and then passing the liquid to yield awet paper composition. The wet composition, once formed on the support,is usually further dewatered by vacuum or other pressure forces andfurther dried by evaporating the remaining liquid.

In one preferred embodiment, the fiber and the fibrids can be slurriedtogether to form a mix that is converted to paper on a wire screen orbelt. Reference is made to U.S. Pat. Nos. 4,698,267 and 4,729,921 toTokarsky; 5,026,456 to Hesler et al.; 5,223,094 and 5,314,742 toKirayoglu et al for illustrative processes for forming papers fromaramid fibers and aramid fibrids.

Once the paper is formed, it is calendered to achieve the final desireddensity and thickness. An optional final step in the paper manufacturingcan include a surface treatment of the paper in a corona or plasmaatmosphere to further improve mechanical properties of the corestructure.

Floc is generally made by cutting continuous spun filaments intospecific-length pieces. Preferably, the floc length is from 2 to 25millimeters. If the floc length is less than 2 millimeters, it isgenerally too short to provide a paper with adequate strength; if thefloc length is more than 25 millimeters, it is very difficult to formuniform wet-laid webs. Floc having a diameter of less than 5micrometers, and especially less than 3 micrometers, is difficult toproduce with adequate cross sectional uniformity and reproducibility; ifthe floc diameter is more than 20 micrometers, it is very difficult toform uniform papers of light to medium basis weights.

Fibrids are typically made by streaming a polymer solution into acoagulating bath of liquid that is immiscible with the solvent of thesolution. The stream of polymer solution is subjected to strenuousshearing forces and turbulence as the polymer is coagulated. The fibridmaterial of this invention can be selected from meta or para-aramid orblends thereof.

Aramid Fiber

P-aramid fiber in the paper of cell wall of the honeycomb of thisinvention can be in the form of the cut fiber (floc), in the form of thepulp or a blend thereof. Floc comprises short fibers made by cuttingcontinuous filament fibers into short lengths without significantfibrillation. Short reinforcing fibers suitable for use in the presentinvention are those disclosed in U.S. Pat. No. 5,474,842 to Hoiness.

The term “pulp”, as used herein, means particles of fibrous materialhaving a stalk and fibrils extending generally therefrom, wherein thestalk is generally columnar and 10 to 50 micrometers in diameter and thefibrils are fine, hair-like members generally attached to the stalkmeasuring only a fraction of a micrometer or a few micrometers indiameter and 10 to 100 micrometers long.

The term “fibrids” as used herein, means a very finely-divided polymerproduct of small, filmy, essentially two-dimensional, particles knownhaving a length and width of 100 to 1000 micrometers and a thickness of0.1 to 1 micrometer.

As employed herein the term aramid means a polyamide wherein at least85% of the amide (—CONH—) linkages are attached directly to two aromaticrings. Additives can be used with the aramid. In fact, it has been foundthat up to as much as 10 percent, by weight, of other polymeric materialcan be blended with the aramid or that copolymers can be used having asmuch as 10 percent of other diamine substituted for the diamine of thearamid or as much as 10 percent of other diacid chloride substituted forthe diacid chloride of the aramid. Para aramid fibers and various formsof these fibers are available from E. I. du Pont de Nemours and Company,Wilmington, Del. under the trademark Kevlar® and from Teijin, Ltd.,under the trademark Twaron®.

Compression Enhancing Layer

The term “compression enhancement layer” as used herein, means anysubstance, which can be applied on the surface of the paper in anysuitable quantity up to and including 5 weight percent of the paperweight. The material of the compression enhancement layer can be appliedto the paper at any convenient stage in the honeycomb making processprior to applying the matrix resin coating. For example, in the case ofan expansion process for making honeycomb, the compression enhancementlayer can be applied to the paper prior to printing node lines. In thecase of a corrugation process for making honeycomb, the compressionenhancement layer can be applied to the paper prior to corrugation. Inanother alternative, the compression enhancement layer can be applied tothe paper after the honeycomb shape has been formed irrespective of thehoneycomb making process.

Any suitable material can be used for the compression enhancement layerdepending on the paper composition, type of the matrix resin etc. Oneexample of a suitable material is an epoxy coating based onglycerintriglycidylether. Another example is a polypeptideprotein-carbon nanotube complex such as is described in PCT patentapplication publication WO2011/027342 to Wolf et al., or apolypeptide-graphite complex. Coupling agents and primers can also beused as compression enhancing layer materials in the current invention.

The compression enhancement layer can be applied in any suitablequantity from about 0.1% to about 5% of the weight of the paper,depending on the particular type of materials used in this layer.

Test Methods

Paper density was calculated using the paper thickness as measured basedon ASTM D374-99 (Reapproved 2004) using a foot pressure of 0.9 kPa andthe basis weight was measured by ASTM D646-96 (Reapproved 2001). Fiberdenier is measured using ASTM D1907-07.

Gurley Porosity for papers was determined by measuring air resistance inseconds per 100 milliliters of cylinder displacement for 6.4 squarecentimeters circular area of a paper using a pressure differential of1.22 kPa in accordance with TAPPI T460 om-96.

The density of the core was measured in accordance with ASTMC271/C271M-05 and the stabilized compression strength was measured inaccordance with ASTM C365/C365M-05.

EXAMPLES Example 1

A p-aramid fiber paper comprised of Kevlar® 49 floc and Nomex® fibridswas formed on conventional paper forming equipment. The composition ofthe paper was 60 weight % Kevlar® floc and 40 weight % Nomex® fibrids.The Kevlar® floc had a nominal filament linear density of 1.5 denier perfilament (1.7 dtex per filament) and a 6.4 mm cut length. The Nomex®fibrids were made as described in U.S. Pat. No. 3,756,908 to Gross.

The paper was then calendered under a linear pressure of 2600 N/cm at atemperature of 330° C. This produced the final paper with a thickness of38 micrometers, a density of 0.75 g/cm³ a basis weight of 0.9 oz/yd²(33.9 g/m²), and Gurley air resistance of 700 seconds per 100milliliters.

The paper was treated in-line with a water based emulsion comprisingDenacol® EX-313 resin (glycerolpolyglycidylether), Aerosol OT, sodiumcarbonate, and soft water in amounts of 17.8, 0.11, 0.15 and 72 percentby weight respectively to form a compression enhancement layer. Denacol®EX-313 was supplied by Nagase ChemteX Corporation, Aerosol OT by FitzChem Corp., and sodium carbonate by Fisher Scientific (ChemicalsDivision). The total quantity of the compression enhancement layer was1.2 weight percent based on the weight of the paper.

A honeycomb was then formed from the treated paper. Node lines ofsolvated adhesive were applied to the paper surface at a width of 2 mmand a pitch of 5 mm and the solvent removed.

The sheet with the adhesive node lines was cut into 500 mm lengths. Aplurality of sheets were stacked one on top of the other, such that eachof the sheets was shifted to the other by half a pitch or a half theinterval of the applied adhesive node lines. The shift occurredalternately to one side or the other, so that the final stack isuniformly vertical. The number of stacked sheets was then hot-pressedbetween plates at the softening point of the adhesive, causing theadhesive node lines to flow. Once the heat was removed the adhesivehardened to bond adjacent sheets to each other. The bonded aramid papersheets were then expanded in the direction counter to the stackingdirection to form cells having an equilateral cross section. Each of thesheets were extended between each other such that the sheets were foldedalong the edges of the bonded node lines and the portions not bondedwere extended in the direction of the tensile force to separate thesheets from each other. A frame was used to expand and hold thehoneycomb in the expanded shape. The expanded cell size was 3.2 mm.

The expanded honeycomb was then placed in a bath containingsolvent-based MIL-R-9299C standard phenolic resin. The phenolic resinwas used in a liquid form wherein the resin was dissolved in ethanol.The resin adhered to and coated the interior surface of the cell wallsas well as penetrating into the pores of the paper. After impregnatingwith resin, the honeycomb was taken out from the bath and was dried in adrying furnace by hot air to remove the solvent and cure the phenolicresin. The impregnation step in the resin bath and the drying step inthe drying furnace were repeated a further two times. The honeycomb corehad properties as shown in Table 1.

Comparative Example 1

A p-aramid fiber paper comprised of Kevlar® 49 floc and Nomex® fibridswas formed on conventional paper forming equipment. The composition ofthe paper was 73 weight % Kevlar® floc and 27 weight % Nomex® fibrids.The Kevlar® floc had a nominal filament linear density of 1.5 denier perfilament (1.7 dtex per filament) and a 6.4 mm cut length. The Nomex®fibrids were made as described in U.S. Pat. No. 3,756,908 to Gross. Thepaper was then calendered under a linear pressure of 2600 N/cm at atemperature of 330° C. This produced the final paper with a thickness of38 micrometers, a density of 0.75 g/cm³ a basis weight of 0.9 oz/yd²(33.9 g/m²), and Gurley air resistance of 15 seconds per 100milliliters. The paper was treated in-line to form a compressionenhancement layer as in Example 1.

A honeycomb was then formed from the treated paper in the same way asdescribed in Example 1. The core had properties as shown in Table 1.

Comparative Example 2

The paper and honeycomb was prepared as in Example 1 with the exceptionthat there was no compression enhancement layer. The core had propertiesas shown in Table 1.

As can be seen from the data of Table 1 and FIG. 4, for the same coredensity, the honeycomb core of this invention has surprisingly highercompression strength in comparison with the cores from the bothcomparative examples.

Example 2

The paper was made as in Example 1 with the exception that it wascalendered to higher density of 0.85 g/cm³. A compression enhancementlayer was coated onto the paper as in Example 1 and a honeycomb coremade as previously described. The properties of this core are also shownin Table 1.

TABLE 1 Exam- Compara- Compara- Exam- ple 1 tive 1 tive 2 ple2 PaperBasis Weight (g/m²) 33.9 33.9 33.9 33.9 Paper density, g/cm3 0.75 0.750.75 0.85 Paper Gurley Air Resistance 700 15 700 1500 (seconds/100 ml)Compression enhancement Yes Yes No Yes layer present Honeycomb CellSize, (mm) 3.2 3.2 3.2 3.2 Core Density (kg/m³) 40 40 40 40 StabilizedCompression 2.14 1.65 1.70 2.30 Strength of the Honeycomb Core (MPa)

As can be seen from the data in Table 1, to get a significantimprovement in compression strength of the honeycomb core from ap-aramid paper, it is insufficient to use a paper with correct level ofGurley air resistance (Comparative Example 2) or to use a paper having acompression enhancement layer (Comparative Example 1). Only thecombination of both the features of the correct right level of Gurleyair resistance and a compression enhancement layer as in Examples 1 and2 provides a significant improvement in compression strength, in thiscase about 20 and 35 percent respectively.

What is claimed:
 1. A method of making a fiber-reinforced compositestructure comprising the steps of: (i) forming a paper sheet having aGurley air resistance at least 200 seconds per 100 milliliters, thesheet comprising from 30 to 70 weight percent p-aramid fiber, (ii)depositing on both surfaces of the paper sheet a compression enhancinglayer, comprising glycerolpolyglycidylether, polypeptide-carbon nanotubecomplexes, polypeptide-graphite complexes or combinations thereof, in aquantity up to 5 weight percent based on the weight of the paper, (iii)forming a honeycomb from the sheet of step (ii), and (iv) applying amatrix resin coating of phenolic, polyimide, polyetherimide, epoxy orcombinations thereof to the honeycomb of step (iii).
 2. The method ofclaim 1 wherein the wherein the paper comprises meta-aramid fibrids,para-aramid fibrids or combinations thereof.
 3. The method of claim 1wherein the paper density is greater than 0.8 g/cm³.
 4. The method ofclaim 1 wherein the matrix resin is phenolic and the compressionenhancing layer comprises glycerolpolyglycidylether.